Method and apparatus for performing random access in a mobile communication system

ABSTRACT

The present disclosure relates to a method and apparatus for performing random access in a user equipment for a small cell e-NB with a small cell service area in heterogeneous e-NB cell carrier integration (dual connectivity or inter-eNB carrier aggregation) in mobile communication systems. In accordance with an aspect of the present disclosure, a method for performing random access in a mobile communication system is provided. The method includes receiving a configuration request message for configuring a Serving Cell Group (SCG) from a second eNB located in a service area of a first eNB through the first eNB; configuring an SCG cell based on the configuration request message, and sending a configuration response message in response to the configuration request message to the second eNB through the first eNB; and performing random access if there is uplink data present on a logic channel (LCH) relocated into the SCG cell.

PRIORITY

This application claims priority under 35 U.S.C. § 119(a) to a KoreanPatent Application filed in the Korean Intellectual Property Office onJan. 29, 2014 and assigned Serial No. 10-2014-0011267, the entiredisclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a method and apparatus for performingrandom access in a user equipment for a small cell e-NB with a smallcell service area in heterogeneous e-NB cell carrier integration (dualconnectivity or inter-eNB carrier aggregation) in mobile communicationsystems.

BACKGROUND

In general, mobile communication systems have been developed to allowthe user to communicate while moving around. The mobile communicationsystem, fueled by rapid development of technologies, has reached a stageof providing high-speed data communication services as well as voicecommunication. In recent years, as one of next generation mobilecommunication systems, Long Term Evolution-Advanced (LTE-A) is beingstandardized by the 3rd Generation Partnership Project (3GPP). LTE-A isa technology to integrate cells with different frequencies supported inan e-NB to implement high-speed packet based communication that hashigher transmission rate than a data rate currently provided, i,e., acarrier aggregation technology. As the 3GPP standard evolves, a smallcell e-NB for supporting a cell with a small service area in the mobilecommunication system has often been used to expand capacity andeliminate shadow areas. The Release 12 (Rel-12) of the communicationstandard proposes a standard for providing high-speed communicationservices by integrating a cell of the small cell e-NB and a cell of theexisting large cell e-NB, i.e., dual connectivity or inter-eNB carrieraggregation. Unlike the carrier aggregation provided by integrating aplurality of cells in a single e-NB as provided by LTE-A, high-speeduplink/downlink data services may be provided for terminals byintegrating a plurality of cell carriers of heterogeneous e-NBs,according to the Rel-12.

The double connection has connectivity to two different e-NBs at thesame time unlike the existing Rel-10 carrier aggregation, and may thushave the respective independent Physical Uplink Control Channels(PUCCHs). In the existing Rel-10 carrier aggregation, to send controlinformation such as Channel Quality Indicator (CQI) or a response todata received at the UE (e.g., acknowledgment ACK/NACK), a controlchannel of a primary cell (PCell) is shared by secondary cells (SCells).Accordingly, it has a structure in which ACK for downlink data of thecarrier aggregated SCells is delivered on an uplink control channel ofthe PCell. As for a random access channel, in the existing carrieraggregation, since the PCell and the Scell are serving cells included inthe same e-NB, they may use the same uplink timing that the UE has.Accordingly, separate random access procedures for the SCell does notneed to be performed. However, as for carrier aggregation for a distantsmall cell using the Remote Radio Head (RRH) as proposed by the Rel-11,it may follow the existing carrier aggregation scheme in which a smallcell and a macro cell are included in the same e-NB, but requiresseparate random access because the distances from the UE to the macroserving cell and the distant small cell are different. In this regard,in the LTE Rel-11 standard, UE is instructed to configure a TimingAdvance Group (TAG) based on the uplink timing for a serving cell and tocontrol the uplink timing with Medium Access Control (MAC) controlelements (CEs) for each TAG. Likewise, even in carrier aggregationbetween heterogeneous e-NBs in the Rel-12, a small cell and a macro cellare likely to have different uplink timing. However, since the smallcell e-NB may have an uplink control channel in the carrier aggregationbetween heterogeneous e-NBs, the UE may control the uplink timing forserving cells of the small cell e-NB through a direct random accessprocedure. Therefore, definition for a random access procedure for aserving cell of a small cell e-NB in a mobile communication system isrequired.

SUMMARY

The present disclosure provides a method and apparatus for performingrandom access for a serving cell (or a Secondary Cell Group (SCG)) of asmall cell eNB.

In accordance with an aspect of the present disclosure, a method forperforming random access in a mobile communication system is provided.The method includes receiving a configuration request message forconfiguring a Serving Cell Group (SCG) from a second eNB located in aservice area of a first eNB through the first eNB; configuring an SCGcell based on the configuration request message, and sending aconfiguration response message in response to the configuration requestmessage to the second eNB through the first eNB; and performing randomaccess if there is uplink data present on a logic channel (LCH)relocated into the SCG cell.

In accordance with another aspect of the present disclosure, a methodfor performing random access in a mobile communication system isprovided. The method includes receiving a Serving Cell Goup (SCG)addition request message from a first eNB at a second eNB located in aservice area of the first eNB; sending a configuration request messageto a UE through the first eNB in response to the SCG addition requestmessage; and receiving a configuration response message through thefirst eNB in return for the configuration request message, andperforming random access with the UE.

In accordance with another aspect of the present disclosure, anapparatus for performing random access in a mobile communication systemis provided. The apparatus includes a receiver for receiving aconfiguration request message for configuring a Serving Cell Group (SCG)from a second eNB located in a service area of a first eNB through thefirst eNB; a controller for configuring an SCG cell based on theconfiguration request message, controlling a configuration responsemessage in response to the configuration request message to be sent tothe second eNB through the first eNB, and performing random access ifthere is uplink data present on a logic channel (LCH) relocated into theSCG cell; and a transmitter for sending the configuration responsemessage under control of the controller.

In accordance with another aspect of the present disclosure, anapparatus for performing random access in a mobile communication systemis provided. The apparatus includes a receiver for receiving a ServingCell Goup (SCG) addition request message from a first eNB at a secondeNB located in a service area of the first eNB; a transmitter forsending a configuration request message to a UE through the first eNB inresponse to the SCG addition request message; and a controller forperforming random access with the UE if a configuration response messagethrough the first eNB is received by the receiver in return for theconfiguration request message.

Other aspects, advantages, and salient features of the disclosure willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present disclosurewill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a schematic diagram illustrating a mobile communicationsystem, according to an embodiment of the present disclosure;

FIG. 2 shows a radio protocol architecture in a mobile communicationsystem, according to an embodiment of the present disclosure;

FIG. 3 shows deployment of small cell eNBs in a mobile communicationsystem, according to an embodiment of the present disclosure;

FIG. 4 is a message flowchart illustrating a procedure of offloadinguser data to a small cell eNB, according to an embodiment of the presentdisclosure;

FIGS. 5A and 5B are message flowcharts of a random access procedure in amobile communication system, according to an embodiment of thedisclosure;

FIG. 6 is a message flowchart of a random access procedure in a mobilecommunication system, according to another embodiment of the disclosure;

FIG. 7 is a flowchart illustrating a method for performing random accessin UE of a mobile communication system, according to an embodiment ofthe present disclosure;

FIG. 8 is a flowchart illustrating a method for performing random accessin UE of a mobile communication system, according to another embodimentof the present disclosure;

FIG. 9 is a flowchart illustrating a method for performing random accessin an SeNB of a mobile communication system, according to an embodimentof the present disclosure;

FIG. 10 is a flowchart illustrating a method for performing randomaccess in an SeNB of a mobile communication system, according to anotherembodiment of the present disclosure; and

FIG. 11 is a block diagram of an apparatus for performing random accessof UE or eNB in a mobile communication system, according to anembodiment of the present disclosure.

Throughout the drawings, like reference numerals will be understood torefer to like parts, components, and structures.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings, wherein like referencenumerals refer to the like elements throughout. Descriptions of somewell-known technologies that possibly obscure the invention will beomitted, if necessary.

Problems to be solved will be first described in connection withembodiments of the present disclosure, and then a method and apparatusfor performing random access in a mobile communication system will bedescribed in accordance with embodiments of the present disclosure.

In the technology of carrier aggregation between heterogeneous eNBs ofthe Rel-12, random access for a serving cell group or a Secondary CellGroup (SCG) of a small cell eNB may be performed, so a need exists todefine the procedure. In the existing random access technology, whenuplink transmission is attempted in handover to send a Radio ResourceControl (RRC) connection reconfiguration complete(rrcConnectionReconfigurationComplete) message, random access isinitiated in a target cell by triggering a regular Buffer Status Report(BSR). In other words, since the UE has no uplink sync information forthe target cell, a random access procedure is required to send thehandover message to the eNB.

In the Rel-12 inter-eNB carrier aggregation, when a macro eNB isreported of measurement information of surrounding small cells from aterminal to add a serving cell of a small cell eNB for the first time,the macro eNB performs a cell addition procedure by selecting a smallcell based on the report. In the Rel-12 inter-eNB carrier aggregation ordouble connection, the RRC functionality of the UE is operated only inthe macro eNB. An RRC message of the UE is thus sent to the macro eNB,and configuration of the small cell eNB is sent using an Xn or X2interface between the macro eNB and the small cell eNB. Thus, since therrcConnecitonReconfigurationComplete message is sent through a MasterCell Group (MCG) of the macro eNB rather than the small cell eNB if anSCG cell is added, it is impossible to trigger random access for the SCGby means of the RRC control message. Sending some part of the controlmessage to the MCG and some other part to the SCG may significantlyincrease complexity, so the RRC message is only sent to the macro eNB.Accordingly, in order to perform a random access procedure for uplinktiming synchronization to the target eNB, i.e., the small cell eNB as inthe existing handover procedure, a different procedure rather than theexisting RRC message is needed. Furthermore, since the RRC functionalityof the UE is in the macro eNB (MeNB), if SCG resource reconfiguration,e.g., addition and reconfiguration of a primary SCell (pSCell, a specialcell among SCG cells), reconfiguration of PUCCH uplink control channel,or reconfiguration of security is required, the small cell eNB (SeNB)sends the rrcConnectionReconfiguration message to the UE through theMeNB. However, due to transmission delay between the two eNBs, when toreconfigure the radio connection and when to initiate the configurationis unpredictable. If the UE and the SeNB hold different radio resourceconfigurations, failure in communication may occur. Thus, a scheme tosynchronize RRC configurations of the UE and SeNB through random accessin the SCG is needed.

To solve the problem, a method and apparatus for performing randomaccess in a mobile communication system in accordance with embodimentsof the present disclosure will now be described in detail.

FIG. 1 is a schematic diagram illustrating a mobile communicationsystem, according to an embodiment of the present disclosure. A LongTerm Evolution (LTE) system is shown in FIG. 1, for example.

Referring to FIG. 1, a radio access network of the LTE system includesevolved Node Bs (hereinafter, also referred to as ENBs, Node Bs, or BaseStations (BSs)) 105, 110, 115, 120, a Mobility Management Entity (MME)125, and a Serving Gateway (S-GW) 130. A UE or terminal 135 access anexternal network via the ENB 105, 110, 115, 120 and the S-GW 130. ENBs105, 110, 115, 120 of FIG. 1 corresponds to an existing Node B of aUniversal Mobile Telecommunications System (UMTS) system. The ENB isconnected to the UE 135 via a wireless channel, and plays a moresophisticated role than the existing node B does. In the LTE system, alluser traffic including real-time services, such as Voice over IP (VoIP)services according to an Internet Protocol is served on a sharedchannel, so a device to collect status information, such as bufferstatus of UEs, available transmission power status, channel status,etc., and schedule the resources is required, and the ENB 105, 110, 115,120 serves as the device. A single ENB may typically control a number ofcells. To achieve 100 Mbps of transmission speed, the LTE system usesOrthogonal Frequency Division Multiplexing (OFDM) in the bandwidth of 20MHz for a radio access technology. It also employs an AdaptiveModulation & Coding (AMC) scheme that determines a modulation scheme andchannel coding rate based on the channel status of the UE. The S-GW 130is a device to provide a data bearer, producing or eliminating the databearer under control of the MME 125. The MME 125 is a device responsiblefor various control operations as well as mobility managementfunctionality for the UE, and connected to a number of eNBs.

FIG. 2 shows a radio protocol architecture in a mobile communicationsystem, according to an embodiment of the present disclosure.

Referring to FIG. 2, the radio protocol of a mobile communication systemincludes Packet Data Convergence Protocol (PDCP) 205, 240, Radio LinkControl (RLC) 210, 235, and Medium Access Control (MAC) 215, 230 in eachof the UE and ENB. The PDCP 205, 240 is in charge of operations like IPheader compression/restoration, and the RLC 210 is in charge ofreconfiguring a PDCP Packet Data Unit (PDU) into a suitable size andperforming e.g., Automatic Repeat reQuest (ARQ) operation. The MAC 215,230 is connected to a number of RLC layer devices configured in a singleUE, for multiplexing RLC PDUs to a MAC PDU and dimultiplexing RLC PDUsfrom a MAC PDU. Physical layer (PRY) 220, 225 performs channel codingand modulation on upper layer data, forms the data into OFDM symbols andsends them on a radio channel, demodulates OFDM symbols received on aradio channel, performs channel decoding and sends the result to anupper layer.

FIG. 3 shows deployment of small cell eNBs in a mobile communicationsystem, according to an embodiment of the present disclosure.

Referring to FIG. 3, a strategy for deploying a small cell eNB 305 is tosparsely deploy the small cell eNB 305 within a service area of a macrocell eNB 300. Such deployment is useful in covering a hot spot areawhere a lot of traffic occurs intensively, or a service shadow area. Thesmall cell eNB 305 may use a frequency band the same as or differentfrom that of the macro cell eNB 300.

Another strategy for deploying the small cell eNB 305 is to denselydeploy a plurality of small cell eNBs 310, 315, 320, . . . ,340 to beadjacent to each other. This may be applied when a lot of traffic occursin a rather wide area. The small cell eNBs 310 to 340 may even be in theservice area of the macro cell eNB 300, or may not be in the servicearea of the macro cell eNB 300 because the service area may be occupiedonly by the small cell eNBs 310-340. All the small cell eNBs 310-340 mayuse the same frequency band, or may use different frequency bands. Thesmall cell eNBs 310-340 may be mainly used to offload the traffic of themacro cell eNB 300. Once the UE measures signals of the surroundingsmall cell eNBs 310-340 and informs the macro cell eNB 300 of themeasurements, the macro cell eNB 300 determines whether to offload userdata to any of the small cell eNB 310-340.

FIG. 4 is a message flowchart illustrating a procedure to offload userdata to a small cell eNB, according to an embodiment of the presentdisclosure. In this message flowchart, respective messages may usedifferent names, but if some of them include similar information, theymay be considered to be the same.

Referring to FIG. 4, a UE 401 measures signals of nearby small cell andmacro cell, and reports a macro eNB (or called MeNB) 403 of themeasurement results in a measurement report. The measurement report is afunction performed by the UE, specified in the existing LTE standard,which is a procedure to report results of measuring neighboring signalsunder the rule defined by the eNB. With the measurement reportprocedure, the eNB may perform a handover procedure for the UE orproperly schedule the eNB radio resources. This is defined as RadioResource Management (RRM). Double connection may be made in aheterogeneous network in which a macro cell and small cells are mixed asshown in FIG. 1. Double connection refers to respective connection todifferent eNBs, and eNBs that corresponds to the double connection arethe MeNB 403 and SeNB 405 of FIG. 4. In other words, the UE 401 may makeconcurrent connections to the MeNB 403 and SeNB 405. In the doubleconnection situation, the MeNB 403 manages RRM of the UE. Accordingly,the UE 401 periodically reports the MeNB 403 of measurements of a nearbycell. Furthermore, the MeNB 403 is periodically reported from the SeNB405 of information about resource usage status of the small cell eNB,e.g., an extent of Physical Resource Block (PRB) usage and transmissionoutput, in a resource status update message, in operation 409. With thereport, the MeNB 403 determines whether to use the SeNB 405 as anassisted cell. Once the MeNB 403 determines to add the SeNB 405 based onthe information, it sends a SCellCommand message to the SeNB 405 throughan X2 interface, in operation 411. The SCellCommand message includeswhat is about addition or release of a serving cell (SCell or secondarycell) belonging to the SeNB. Requested for addition or release, the SeNB405 sends the MeNB 403 information about RRC reconfiguration in aSCellConfig message, in operation 413. The SCellConfig message includesinformation for RRC reconfiguration about addition or release of theserving cell of the SeNB. The SCellConfig message may be an RRC messagedefined in the existing LTE standard, or information for an RRC messageto be created by the MeNB 403. If the SCellConfig message is the RRCmessage itself for the UE 401, the MeNB 403 simply forwards the messageto the UE 401. Otherwise, the MeNB 403 creates an RRC reconfigurationmessage based on the SCellConfig message. Operations 415 to 417represent a procedure to send the RRC reconfiguration message for newlyadding or releasing a serving cell of the SeNB 405 and receive aresponse message. The message sending procedure conforms to the existingLTE standard. With the procedure, the UE 401 performs an accessprocedure or a release procedure for the SeNB 405 in operations 415 to417. The SCellConfig message includes information about a specificserving cell able to perform PUCCH or Random Access Channel (RACH). Theserving cell is called a primary Scell, pSCell, which is different fromother serving cells (SCells) of the SeNB. Other SCells have no PUCCH orRACH, and ACK for downlink data is sent through the pSCell. Uponreception of the RRC message, the UE 401 performs a random accessprocedure to the pSCell to add the serving cell of the SeNB 405 for thefirst time, in operation 415. The random access procedure is a procedurefor uplink synchronization and designation of an uplink transmissionoutput for the serving cell of the SeNB 405, and a procedure to requestinitial uplink radio resources. The respective MeNB 403 and SeNB 405then send ACK in response to the received message, in operations 419 and421.

For offloading to a small cell in FIG. 4, the MeNB 403 needs signalstrength values of surrounding serving cells measured by the UE 401 andsmall cell eNB status information sent from the SeNB 405, in operation409, in order to add the serving cell of the SeNB 405. The two valuesare not relevant to each other, and the UE 401 has no information aboutthe resource status update message.

FIGS. 5A and 5B are message flowcharts of a random access procedure in amobile communication system, according to an embodiment of thedisclosure. FIG. 5A shows the MAC of a UE initiating a random accessprocedure in a pSCell of the SCG, and FIG. 5B shows an eNB requesting arandom access procedure.

In the existing Rel-11 carrier aggregation, there is no procedure wherethe UE performs random access to an SCell as shown in FIG. 5A, but onlya procedure where a request is sent to the UE in a dedicated preamble onthe Physical Downlink Control Channel (PDCCH) to perform a random accessprocedure in a PCell or pSCell as shown in FIG. SB. On the other hand,in the Rel-12 heterogeneous carrier aggregation or double connection,both procedures of FIGS. 5A and 5B may be performed for respectivePCells of the MeNB and SeNB.

Referring to FIG. 5A, MeNB 510 sends an initial request for SCGconfiguration to UE 505 in the rrcConnectionReconfiguration message foraddition of a pSCell, in operation 520. The UE 505 activates its radiotransceiver based on the rrcConnectionReconfiguration message andsynchronizes the system clock to a downlink sync signal for the pScell,in operation 525. As such, the UE 505 completes downlink syncconfiguration for the SCG of the SeNB 515. The UE 505 then performs arandom access to the pSCell 515 on the RACH, in operation 530. In thisregard, in contrast to Rel-10, contention based random access with arandom preamble is possible. Alternatively, in a case therrcConnectionReconfiguration received from the MeNB 510 includes adedicated preamble delivered from the SeNB 515 of the SCG,non-contention based random access with the preamble is performed.

Referring to FIG. 5B, upon reception of the rrcConnectionReconfigurationmessage for addition of SeNB 550 of the SCG for the first time, UE 540performs downlink signal synchronization to the SCG pSCell, in operation560. After completing the downlink signal synchronization, the UE 540sends a response message (rrcConnectionReconfigurationComplete message)to the RRC message to the MeNB 545, in operation 565. The MeNB 545 thensends the SeNB 550 ACK for reception of therrcConnectionReconfigurationComplete message, in operation 570. The SeNB550 recognizes that configuration of the UE is completed by RRC, andsends the UE a random access request on the PDCCH for establishment ofuplink timing, in operation 575. After performing the random accessprocedure, the UE 540 secures uplink synchronization for the SCG.

The following Table 1 represents scenarios in which Random Access (RA)is required and schemes for requesting to perform random access in theexisting carrier aggregation. In Table 1, to access a first network, MACperforms contention based random access that requests random access tothe PCell in the idle state. A procedure to reconfigure RRC connectionis also the same as a scheme required in the initial access procedure.As for handover, as a procedure to obtain uplink radio resources to senda handover complete message to a target eNB, non-contention based randomaccess is performed with a dedicated preamble of the target eNB cellreceived through a source eNB. In a situation where data transmission isresumed after there has not been data transmission for a while in theconnected state, the UE might lose uplink timing, so the eNB may sendthe UE a request on the downlink control channel for a random accessprocedure for the PCell or SCell for coordination of uplink timingbefore the eNB resumes downlink data transmission. In contrast, if theeNB is to send uplink data, it attempts random access at the MACrequest. Lastly, PCell or SCell requests random access on the downlinkchannel for the positioning purpose while connected.

TABLE 1 PDCCH/MAC RA triggering events in 36.300 initiated PCell/SCellInitial access from RRC_IDLE MAC initiated PCell RRC connectionRe-establishment MAC initiated PCell procedure Handover MAC initiatedPCell DL data arrival during PDCCH initiated PCell/SCell RRC_CONNECTEDUL data arrival during MAC initiated PCell RRC_CONNECTED For positioningpurpose during PDCCH initiated PCell/SCell RRC_CONNECTED

Comparing the flows of pSCell random access upon request of the MAClayer and random access triggered by the small cell eNB, SeNB, as shownin FIGS. 5A and 5B, respectively, the random access procedure triggeredby the MAC of the UE as shown in FIG. 5A is simpler. Accordingly,complexity of the procedure may be reduced by enabling random access inthe SeNB pSCell.

The following Table 2 represents random access procedures and types thatmay occur in a situation of double connection or inter-eNB carrieraggregation in accordance with an embodiment of the present disclosure.

Similar to the existing carrier aggregation, contention based ornon-contention based random access is possible in a pSCEll or SCell ofSCG. Start of random access on the downlink control channel in MAC oreNB is all enabled in PCell in the existing carrier aggregation, andrandom access is enabled only on the downlink control channel in anSCell. In the Rel-12 double connection, random access may be initiatedby both schemes as a PCell of an MCG for the pSCell, and random accessmay be initiated only by the downlink control channel as an SCell of theMCG for the SCell.

TABLE 2 PCell SCell in MCG pSCell SCell in SCG Contention based or BothBoth Both Both Contention free random access MAC initiated Both OnlyPDCCH Both Only random access or initiated RA PDCCH PDCCH initiatedinitiated RA random access Note As per up to As per Rel-11 Aligned withRel-11 Rel-11 principles

The following Table 3 represents operations of the MAC layer for eventsthat trigger the random access. The description is denoted in the LTEstandard TS 36.300. First, when the UE is changed into a connected statefrom idle state, the rrcConnectionRequest message requesting an accessto a network is sent from the upper RRC layer on the Common ControlChannel (CCCH). This induces Regular Buffer Status Report (R-BSR), andtriggers a random access procedure to send an R-BSR MAC CE. Secondly,when the UE has lost its radio connection temporarily, as a procedure tore-establish the connection, the rrcConnectionReestablishmentRequestmessage is sent on the CCCH, which immediately leads to triggeringtransmission of R-BSR to send the uplink message, and a random accessprocedure is performed to send the R-BSR MAC CE. In case of handover, tosend an rrcConnectionReconfigurationComplete message to a target eNB,transmission of the R-BSR is requested, for which random access istriggered. In case of re-start of downlink transmission, since the eNBrequests random access on the downlink control channel, triggering inthe MAC layer of the UE is not necessary. In resuming uplink datatransmission, B-BSR transmission is requested while uplink data occurson DTCH or DCCH of the UE, which requests random access. Although notspecified in the standard, an occasion where there is no response fromthe eNB even after Scheduling Requests (SRs) have been sent from the UEin connected state more than a number of times determined by the eNB isdefined as SR failure, in which case the UE performs random access tore-establish the connection.

TABLE 3 RA triggering events in PDCCH/MAC 36.300 initiated Triggering in36.321 Initial access from MAC initiated R-BSR for RRC_IDLErrcConnectionRequest RRC connection MAC initiated R-BSR forRe-establishment rrcConnection- procedure ReestablishmentRequestHandover MAC initiated R-BSR for rrcConnection- ReconfigurationCompleteDL data arrival during PDCCH initiated Reception of PDCCH orderRRC_CONNECTED UL data arrival during MAC initiated R-BSR for DCCH/DTCHRRC_CONNECTED For positioning purpose PDCCH initiated Reception of PDCCHorder during RRC_CONNECTED Not captured in 36.300 MAC initiated UponD-SR failure

FIG. 6 is a message flowchart of a random access procedure in a mobilecommunication system, according to another embodiment of the disclosure.In other words, FIG. 6 shows a random access procedure forsimultaneously applying configuration of rrcConnectionReconfiguration infirst SCG addition or reconfiguration of the current SCG cell.

First, for first SCG addition, MeNB 602 sends SeNB 603 a request messageto add a corresponding SCG serving cell, in operation 605, as inoperation 411 of FIG. 4. In response, the SeNB 603 sends an SCG-Configmessage to the MeNB 602, in operation 610. The MeNB 602 then sends UE601 a request to additionally configure the SCG cell in therrcConnectionReconfiguration message, in operation 620. Upon receptionof the SCG-Config message from the SeNB 603, the MeNB 602 may exchangedata and information about SN status with the SeNB 603, in operations630, 640.

After that, upon reception of the rrcConnectionReconfiguration message,the UE 601 responds to the MeNB 602 with arrcConnectionReconfigurationcomplete message. The MeNB 602 then sendsthe SeNB 603 ACK for reception of therrcConnectionReconfigurationComplete message, in operation 660.Accordingly, the UE 601 performs a random access procedure with the SeNB603, in operation 670.

In the meantime, since the rrcConnectionReconfigurationcomplete messageis sent from the UE 601 to the MeNB 602, if the UE 601 has no uplinktransmission data for the SeNB 603 since the message has been sent, arandom access procedure is not triggered in operation 670. Asrepresented in Table 3, since the rrcConnectionReconfiguration messageis sent to the SeNB 603 in the general handover procedure, if themessage is delivered on an uplink data channel (DTCH), the MAC layercreates R-BSR for the message and determines whether the SCG MAC securesuplink transmission sync to the eNB and whether there is a radioresource for uplink data transmission, to send the MAC CE to the SeNB.

In an occasion of first access to the SCG as shown in FIG. 6, sinceuplink sync is not secured for the small cell eNB, random access needsto be performed. On the other hand, if, in FIG. 6, therrcConnectionReconfiguration message has been sent to the MeNB and thereis no data to be transmitted to the SeNB, a random access procedure isnot performed. Accordingly, in FIG. 6, in a random access procedure inaccordance with an embodiment of the present disclosure, SeNB sends theMeNB secondary mobility control information (sMCI), or MCI-SCG, orconfiguration information with a different name in the SCG-Configmessage to be sent for SCG cell configuration. The information includedin the SCG-Config message may include a preamble to be used by the UE601 in a random access procedure 670 and radio resource (e.g., RACH)information for random access. With the preamble and radio resource forrandom access, non-contention based random access may be enabled, andotherwise, contention based random access is performed. Further, sincerandom access resources are allocated by the SeNB, timer for theavailable time and configuration of the resources may be designated. Forexample, if it is designated that only 50 msec from the allocated timeis available, the UE performs random access with the preamble and radioresources within 50 msec from when the UE receives an RRC message, andstops the timer when the random access is successful. If the SeNB 603receives a preamble included in MCI-SCG or sMCI through the randomaccess or successfully receives MAC CE for UE information (e.g., SCGC-RNTI) allocated in rrcConnectionReconfiguration that includes the sMCIfrom the UE in the random access procedure, it may recognize that theprocedure of addition of the SCG serving cell has been successfullycompleted and the RRC configuration was successful in the UE. Themessage flowchart of FIG. 6 may be applied not only in the procedure toadd the initial SCG serving cell for the first time but also inimportant configuration in need of RRC configuration sync between the UEand SeNB, such as reconfiguration of pSCell uplink control channel,pSCell change, security key change, etc. For example, even when the SeNBwants to change a current radio resource for the uplink control channel(PUCCH) of pSCell of the UE, the SeNB sends the MeNB a request to sendthe RRC message to the UE, the request including correspondinginformation, in the SCG-Config process 610 in accordance with the randomaccess procedure of FIG. 6. In the process, however, since the SeNB 603has no information about delay that occurs due to the UE adding the SCGserving cell in operations 610 to 660, the random access process 670 isessentially required. In other words, if the UE has not yet completed aprocess of initializing the RF transceiver even though it received aresponse message in operation 660 for the RRC reconfiguration request ofthe SeNB, sending a control message (DCI) on the downlink controlchannel is not necessary and the chances are high that the UE may notreceive the message. Furthermore, even though numerous random accessprocedures may occur in the connection process where the UE and SeNBexchange data, it is impossible for the eNB to tell them from randomaccesses for uplink data transmission or timing sync. Accordingly, ifthe UE sent the SeNB a particular preamble included in sMCI as proposedin the present disclosure in the random access process 670, the SeNB maybe aware that the uplink control channel reconfiguration requested inthe SCG-Config process 610 was safely completed in the UE, and changethe uplink channel configuration of the corresponding small cell. Randomaccess request with the sMCI may be divided largely into two: a processof triggering random access in the process of initially adding an SCGcell, and a process of triggering random access for configuration syncin response to RRC reconfiguration of the SeNB. The two occasions mayfurther be divided based on whether the dedicated preamble is included.In the random access for RRC configuration sync, delay occurs when theeNB receives a preamble and re-establish radio resources that itactually indicates to the UE. This may depend on implementations of theSeNB, but roughly, tens of milliseconds of delay may occur. Therefore,if the RRC configuration begins when the UE has successfully completedthe random access, i.e., when it received Message 2 (a random accessresponse), there is inconsistency in configuration between the UE andthe SeNB for a while. Accordingly, the SeNB may set a timer to indicatethe UE to start the RRC configuration after the random access in the RRCmessage. For example, if the SeNB sends information about a 20 msectimer in the RRC message, the UE receives Message 2 and starts the 20msec timer; the eNB performs RRC reconfiguration within thecorresponding time; and the UE and SeNB begin the respectiveconfiguration to be in sync with each other.

The following Table 4 is an example of triggering conditions of randomaccess in the SCG defined in the mobile communication system inaccordance with the present disclosure. PCell and MCG PCell conform tothe existing LTE standard, so the detailed description will be omittedherein.

TABLE 4 MCG SCG pSCell SCell SCell PCell (index = y) (index = x) (index= x) Initial access from Yes No No No RRC_IDLE RRC connection Yes No NoNo Re-establishment procedure Handover Yes No No No DL data Yes if Yesif Yes if Yes if arrival during PDCCH PDCCH PDCCH PDCCH RRC_CONNECTEDorder is order is order is order is received received from receivedreceived from MCG SCG and cell from MCG from SCG and cell index is andcell and cell index is 0 preconfigured index is x index is x one UL dataYes if MCG Yes if SCG No No arrival during R-BSR is R-BSR isRRC_CONNECTED triggered triggered and and SR on [SR on PUCCH is PUCCH isnot not configured configured in in PCell pSCell or uplink timing is notestablished in pSCell] For positioning Yes if Yes if Yes if Yes ifpurpose during PDCCH PDCCH PDCCH PDCCH RRC_CONNECTED order is order isorder is order is received received from received received from MCG SCGand cell from MCG from SCG and cell index is and cell and cell index is0 preconfigured index is x index is x one SR failure Yes if SR Yes if SRNo No failure failure occurs occurs at at pSCell PCell

As for a pSCell of an SCG, there is no uplink message to be sent on theCCCH, such as rrcConnectionRequest orrrcConnectionReEstablishmentRequest sent from the upper RRC layer forrequesting initial network connection or reconnection afterdisconnection. The reason is that the SCG does not manage mobility andconnection status of UE in the double connection. It is because RRCexists only in a macro eNB. Thus, in Table 4, in case of the initialconnection and reconnection, random access occurs in a PCell but not ina pSCell. For the same reason, in case of handover, random access occursonly in an MCG PCell. If downlink data transmission of SCG is resumed,the SeNB triggers random access on the downlink control channel. In thisregard, an index for an SCell for random access needs to be set inadvance. If uplink data is generated in the UE, R-BSR for SCG isgenerated, which immediately triggers random access. It is limited to anoccasion where no uplink radio resource request (SR) for uplink datatransmission is established or the uplink transmission timing is out ofsync. In the double connection, respective R-BSRs of MCG and SCG may bedistinctively defined. Considering independence in the double connectionof the MAC, most MAC operations may be independently performed forrespective cell groups. Accordingly, MCG R-BSR is triggered only by dataof a logical channel (LCH) of MCG, and SCG R-BSR is triggered only bydata of an SCG LCH. SCG R-BSR is defined to trigger random access in apSCell, and once SCG is configured for the first time, SCG R-BSR isnaturally triggered because an LCH to be relocated to the SCG and uplinkdata exist. On the other hand, if there are no data available fortransmission on the LCH relocated, or if the SCG is configured first andthe LCH is relocated next, SCG-BSR is not triggered.

As a solution for this, redefine R-BSR triggering conditions in thedouble connection.

First, linking whether to relocate the LCH to R-BSR triggering operatesas follows. First, in case of relocation to an SCG LCH, if the newlyrelocated SCG LCH takes higher priority over an old SCG LCH alreadyrelocated, SCG-BSR is triggered regardless of whether data is present ornot. In case of relocation to an MCG LCH, if a stored amount of dataavailable for transmission on the relocated LCH is equal to or more thana threshold, MCG-BSR is triggered.

Secondly, R-BSR triggering with a default LCG operates as follows.First, if SCG (or pSCell) is newly configured and an SCG LCH is notestablished, the UE generates a default SCG LCG with a certainidentifier, i.e., the default LCG is an empty set. Once the default SCGLCG is generated, the UE triggers SCG R-BSR.

Besides that, for positioning, the SeNB triggers random access on thedownlink control channel. Lastly, if uplink radio resource allocation bymeans of SR of a pSCell is failed, a random access process is performed.As for SCG SCell, it is the same as the existing MCG SCell. If thedefault SCG LCG is generated and the LCH is relocated from MeNB to SeNB,but there is no uplink data on the LCH, SCG R-BSR is to be automaticallytriggered as proposed, or to be triggered when the UE sends an arbitrarydummy packet on the LCH. This gives an advantage of triggering randomaccess without change of the existing LTE standard.

FIG. 7 is a flowchart illustrating a method for performing random accessin UE of a mobile communication system, according to an embodiment ofthe present disclosure. This embodiment corresponds to the embodimentdiscussed with reference to FIG. 5A and Table 4.

Referring to FIG. 7, the UE receives an RRC configuration message, i.e.,RRCconnectionReconfiguration message from a macro eNB, in operation 701.The UE then determines whether the received RRC configuration message isabout SCG addition, in operation 703.

If the RRC configuration message is about the SCG addition, the UEactivates the RF transceiver device for the serving cell, and performsestablishment of the corresponding frequency and bandwidth, in operation705. The UE then determines whether a Logical Channel Group (LCG) hasbeen configured for the SCG, in operation 707. If the LCG has alreadybeen configured, the UE selects a bearer for offloading to the SCG fromamong MCG LCH, and performs an LCH relocation procedure for the selectedbearer, in operation 709. The UE transfers the MCG LCH to the SCG forthe bearer to be relocated to the SCG and establishes a new LCH, setsLCH priority of the bearer for offloading to the SCG, and comparespriority between the LCH and the existing other SCG LCH, in operation711. If the newly generated LCH takes higher priority over the existingSCG LCH, the UE triggers BSR for the SCG regardless of whether there isuplink data on the LCH, in operation 717. The UE then performs randomaccess based on the BSR, in operation 719. Otherwise, if the newlygenerated LCH has equal or lower priority over the existing SCG LCH, theUE monitors uplink data, in operation 713.

In a case the UE adds a serving cell of the SCG for the first time, inoperation 720, the UE establishes a default LCG because there is no LCG,in operation 715. In this regard, even if no data occurs for the defaultLCG, the UE triggers BSR for the SCG, in operation 717. This correspondsto the embodiment of the present disclosure as suggested in Table 4,which may solve the problem that random access is not performed in theinitial configuration of SCG addition because no uplink data occurs. TheUE performs random access using SCG BSR triggered in operation 870, inoperation 719.

On the other hand, in a case that the received RRC message is not aboutthe SCG addition, i.e., it is about a release, the UE stops LCHtransmission/reception to the SCG and performs a procedure to releasethe corresponding data channel, in operation 721. That is, the UEperforms a procedure to transform the SCG LCH for which transmission isstopped, to an MCG, in operation 721. After the MCG relocation of theLCH, the UE compares uplink data on the LCH and a certain threshold setby the eNB, in operation 723. If the uplink data on the LCH is higherthan the threshold, the UE triggers MCG BSR, in operation 725. The UEthen performs random access for the MCG, in operation 727.

FIG. 8 is a flowchart illustrating a method for performing random accessin UE of a mobile communication system, according to another embodimentof the present disclosure. This embodiment corresponds to the embodimentas discussed with reference to FIG. 6.

Referring to FIG. 8, the UE receives an RRCConnectionReconfigurationmessage for SCG configuration from a macro eNB, in operation 801. The UEdetermines whether the received RRCConnectionReconfiguration message isabout initial addition of an SCG serving cell, in operation 803. If themessage is about the initial addition of an SCG serving cell, the UEactivates a pSCell in operation 805, and checks whether there is uplinkdata on an LCH relocated to the SCG in operation 807. If there is uplinkdata on the SCG LCH, the UE triggers SCG BSR in operation 809. The UEdetermines whether an information element like sMCI included in theRRCConnectionReconfiguration message has a preamble and random accessradio resources necessary for random access for the SCG in operation811, and performs non-contention based random access if the informationelement has them in operation 813. Otherwise, if the preamble or theradio resources for random access are not reserved in theRRCConnectionReconfiguration message, the UE performs contention basedrandom access in operation 815.

On the other hand, if the RRCConnectionReconfiguration message is notabout the SCG addition, i.e., the RRCConnectionReconfiguration messagehas already been received for reconfiguration of SCG serving cells,e.g., uplink control channel resource configuration and pSCell change,in operation 803, the UE changes configuration for the SCG in operation817. If synchronization of configuration for the SCG is required betweenthe eNB and the UE, the UE determines whether SMCI includes a preambleor radio resources for random access in operation 819. If the SMCIincludes the preamble or radio resources for random access, the UEperforms random access to the SCG with the resources in operation 821.

The eNB may then confirm that the serving cell configuration has beenapplied for the UE based on the preamble included in the SMCI.

FIG. 9 is a flowchart illustrating a method for performing random accessin an SenB of a mobile communication system, according to an embodimentof the present disclosure.

Referring to FIG. 9, an SeNB receives an SCG addition request messagefor double connection of a cell of the SeNB from an MeNB, in operation901. In response to the received SCG addition request message, the SeNBdetermines whether to do the SCG addition, in operation 903. Ifdetermining to do the SCG addition, the SeNB sends the MeNB informationabout RRC configuration of the SCG that includes the corresponding cellin an SCG configuration message through an X2 interface, in operation905. After that, the SeNB activates a pSCell and waits until the UEcompletes configuration for the SCG, in operation 907. At this time, theUE receives the information about SCG configuration from the MeNB in anRRCConnectionReconfiguration message, and in response, sends aRRCConnectionReconfiguration complete message to the MeNB. The MeNB inturn sends the SeNB a response message informing that RRC configurationhas been completed.

The SeNB then receives the response message from the MeNB that informsthat RRC configuration has been completed for the UE, in operation 909.The SeNB then sends a random access request message for the UE toperform a random access procedure on the PDCCH, in operation 911. Afterthat, the SeNB keeps monitoring the established random access channel,and sends the UE a Random Access Response (RAR) message for allocationof resources for an uplink common control channel, in operation 911. TheSeNB then monitors an uplink channel to receive an uplink datatransmission, in operation 915.

On the other hand, if the SeNB does not want to do the SCG addition inresponse to the received SCG addition request message in operation 903,the SeNB determines to release the SCG, in operation 917. The SeNB thensends an SCG configuration release message to the MeNB, in operation919.

FIG. 10 is a flowchart illustrating a method for performing a randomaccess procedure in an SeNB of a mobile communication system, accordingto another embodiment of the present disclosure. For reference, theflowchart of FIG. 10 is equally applied not only to a pSCell additionprocedure but also to a procedure of RRC configuration in need ofsynchronization between the SeNB and the UE as shown in FIG. 6. Forexample, as for uplink resource change, SeNB security key change, andthe like, RRC reconfiguration is performed in the same procedure as thepSCell addition procedure.

Referring to FIG. 10, an SeNB receives an SCG addition request messagefor double connection of the cell of the SeNB from an MeNB, in operation1001. In response to the received SCG addition request message, the SeNBdetermines whether to do the SCG addition, in operation 1003. Ifdetermining to do the SCG addition or change the SCG, the SeNB sends RRCconfiguration content to the MeNB in an RRC configuration messagethrough an X2 interface, in operation 1005. At this time, the SeNB sendsmobilityControllnfoSCG to the UE, which includes a dedicated preamble,RACH resources, and C-RNTI, in operation 1005. The dedicated preamblemay or may not be included. If the dedicated preamble is not included, acontention based random access procedure is performed.

The SeNB performs a random access procedure and receives preambleinformation on an uplink random access channel, in operation 1009. TheSeNB determines whether the received preamble information corresponds toa dedicated preamble allocated, in operation 1011.

If the received preamble information is not the dedicated preambleallocated, the SeNB puts the uplink channel in sync and sends the UE anRAR message for allocation of resources for an uplink common controlchannel for collision analysis with C-RNTI, in operation 1013. The SeNBthen checks the C-RNTI included in the received message on the allocatedcommon control channel, and determines whether it is allocated by theSeNB for double connection of the UE, in operation 1017.

If the C-RNTI included in the received message is allocated by the SeNBfor double connection of the UE, the SeNB takes the UE for oneindependently connected to the SeNB and operates in the same way as theexisting independent eNB operates, in operation 1019. In other words, ifthe C-RNTI included in the received message is not allocated by the SeNBfor double connection of the UE, the SeNB performs uplink monitoring, inoperation 1019. On the other hand, if the C-RNTI included in thereceived message is identical to what is allocated by the SeNB fordouble connection of the UE, the SeNB adopts the RRC configuration andinitiates allocation of uplink/downlink radio resources for the UE, inoperation 1021.

In the meantime, if the received preamble information corresponds to adedicated preamble allocated, i.e., if the preamble informationcorresponds to a preamble sent to a double connection UE, the SeNB sendsan RAR message for timing coordination, in operation 1015. The SeNB thenadopts the RRC configuration for the UE, establishes uplink/downlinkradio resources, and starts data transmission, in operation 1021.

On the other hand, if the SeNB does not want to do the SCG addition inresponse to the received SCG addition request message in operation 1003,the SeNB determines to release the SCG, in operation 1023. The SeNB thensends an SCG configuration release message to the MeNB, in operation1025.

FIG. 11 is a block diagram of an apparatus for performing random accessof UE or an eNB in a mobile communication system, according to anembodiment of the present disclosure.

UE or an eNB sends/receives data to/from an upper layer 1110 whilesending/receiving control messages through a control message processor1115. In sending a control message or data, the UE or eNB multiplexesthe control message or data with a multiplexer 1105 under control of acontroller 1120, and then sends the resultant control message or datathrough a transmitter 1100. On the other hand, in receiving, the UE oreNB receives a physical signal through a receiver 1100 under control ofthe controller 1120, demultiplexes the received signal with thedemultiplexer 1105, and forwards them to the upper layer 1110 or thecontrol message processor 1115 based on respective message information.The controller 1120 performs random access in accordance with theembodiments of the present disclosure as described above, which aredescribed in connection with FIGS. 7 to 10. The description will beomitted herein.

Several embodiments have thus been described, but it will be understoodthat various modifications can be made without departing the scope ofthe present disclosure. Thus, it will be apparent to those ordinaryskilled in the art that the invention is not limited to the embodimentsdescribed, but can encompass not only the appended claims but theequivalents.

What is claimed is:
 1. A method by a terminal communicating with amaster base station and a secondary base station, the method comprising:receiving, from a master base station, a message to reconfigure asecondary cell group (SCG) associated with a secondary base station, themessage including configuration information related to the SCG;reconfiguring the SCG based on the message; identifying whether theconfiguration information related to the SCG includes mobility controlinformation related to the SCG; starting a timer with a timer valueincluded in the mobility control information related to the SCG based onthe configuration information including mobility control informationrelated to the SCG; starting to synchronize to a downlink of a primarysecondary cell (PSCell) based on the configuration information includingthe mobility control information related to the SCG; initiating a randomaccess procedure on the PSCell based on preamble information included inthe mobility control information related to the SCG based on theconfiguration information including the mobility control informationrelated to the SCG; and transmitting, to the master base station, aresponse message in response to the message, wherein the timer isstopped in response to the random access procedure being completedsuccessfully.
 2. The method of claim 1, wherein initiating the randomaccess procedure comprises, performing contention based random access inresponse to no dedicated preamble or radio resource being reserved forthe random access in the preamble information.
 3. The method of claim 1,wherein initiating the random access procedure comprises, performingnon-contention based random access in response to a dedicated preambleor a radio resource being reserved for the random access in the preambleinformation.
 4. The method of claim 1, wherein the timer value is set bythe secondary base station.
 5. The method of claim 1, wherein performingthe random access procedure comprises, sending specific preamble or MACcontrol element corresponding to SCG C-RNTI to the secondary basestation.
 6. A method by a master base station communicating with aterminal and a secondary base station, the method comprising:transmitting, to a terminal, a message to reconfigure a secondary cellgroup (SCG) associated with a secondary base station, the messageincluding configuration information related to the SCG; performing arandom access procedure on a primary secondary cell (PSCell) based onpreamble information; and receiving, from the terminal, a responsemessage in response to the message, wherein a timer, which is startedwith a timer value included in mobility control information related tothe SCG based on the configuration information including mobilitycontrol information related to the SCG, is stopped in the terminal basedon the random access procedure being completed successfully, and whereinthe terminal initiates the random access procedure on the PSCell basedon the preamble information included in the mobility control informationrelated to the SCG based on the configuration information includingmobility control information related to the SCG, wherein the terminalstarts to synchronize to a downlink of the PSCell based on theconfiguration information including the mobility control informationrelated to the SCG.
 7. The method of claim 6, wherein performing therandom access procedure comprises: performing contention based randomaccess in response to no dedicated preamble or radio resource beingreserved for the random access in the preamble information.
 8. Themethod of claim 6, wherein performing the random access procedurecomprises, performing non-contention based random access in response toa dedicated preamble or a radio resource being reserved for the randomaccess in the preamble information.
 9. A terminal communicating with amaster base station and a secondary base station, the terminalcomprising: a transceiver; and a processor coupled with the transceiverand configured to: receive, from a master base station, a message toreconfigure a secondary cell group(SCG) associated with a secondary basestation, the message including configuration information related to theSCG, reconfigure the SCG based on the message, identify whether theconfiguration information related to the SCG includes mobility controlinformation related to the SCG, start a timer with a timer valueincluded in the mobility control information related to the SCG based onthe configuration information including the mobility control informationrelated to the SCG, start to synchronize to a downlink of a primarysecondary cell (PSCell) based on the configuration information includingthe mobility control information related to the SCG, initiate a randomaccess procedure on the PSCell based on preamble information included inthe mobility control information related to the SCG if the configurationinformation including mobility control information related to the SCG,and transmit, to the master base station, a response message in responseto the message, wherein the timer is stopped in response to the randomaccess procedure being completed successfully.
 10. The terminal of claim9, wherein the processor is further configured to perform contentionbased random access in response to no dedicated preamble or radioresource being reserved for the random access in the preambleinformation.
 11. The terminal of claim 9, wherein the processor isfurther configured to trigger non-contention based random access inresponse to a dedicated preamble or a radio resource being reserved forthe random access in the preamble information.
 12. A master base stationcommunicating with a terminal and a secondary base station, the masterbase station comprising: a transceiver; and a processor coupled with thetransceiver and configured to: transmit, to a terminal, a message toreconfigure a secondary cell group (SCG) which is associated with asecondary base station, the message including configuration informationrelated to the SCG, perform a random access procedure on a primarysecondary cell (PSCell) based on preamble information, and receive, fromthe terminal, a response message in response to the message, wherein atimer, which is started with a timer value included in mobility controlinformation related to the SCG based on the configuration informationincluding the mobility control information related to the SCG, isstopped in the terminal in response to the random access procedure beingcompleted successfully, and wherein the terminal initiates the randomaccess procedure on the PSCell based on the preamble informationincluded in the mobility control information related to the SCG based onthe configuration information including the mobility control informationrelated to the SCG, wherein the terminal starts to synchronize to adownlink of the PSCell based on the configuration information includingthe mobility control information related to the SCG.
 13. The master basestation of claim 12, wherein the processor is further configured toperform contention based random access in response to no dedicatedpreamble or radio resource being reserved for the random access in thepreamble information.
 14. The master base station of claim 12, whereinthe processor is further configured to perform non-contention basedrandom access in response to a dedicated preamble or a radio resourcebeing reserved for the random access in the preamble information.